1. Field of the Invention
The present invention relates to a method and apparatus for image forming, a process cartridge included in the apparatus and toner used for the image forming, and more particularly to a method and apparatus for image forming capable of efficiently collecting toner remaining on image forming components without using a cleaning device for preventing a reproduction of a defective reproduction having a background contamination and dust.
2. Description of the Related Art
An image forming apparatus, such as a copying machine, a facsimile machine, a printing machine, and so forth using an electrostatic transfer method generates a transfer electric field between an image bearing member and an intermediate transfer member that travels in contact with the image bearing member. A toner image formed on a surface of the image bearing member is transferred onto the intermediate transfer member. In some other image forming apparatuses, a transfer electric field is generated between the image bearing member and a recording medium that also travels in contact with the image bearing member. The toner image formed on the surface of the image bearing member is also transferred onto the recording medium. With the electrostatic transfer method, residual toner remains on the surface of the image bearing member after the transferring operation of an image forming process. In a next image forming process, when a laser beam irradiates the image bearing member having the residual toner on the surface thereof, electric charges applied on the areas covered by the residual toner cannot be grounded, resulting in producing a defective image having a white spot, for example.
To prevent such a defective image, a cleaning device for removing residual toner from the image bearing member is disposed at a position opposite to the image bearing member between a transferring area and a charging area so that the residual toner can be removed. However, this requires additional space in the image forming apparatus since the cleaning device includes a toner collecting tank for collecting the residual toner removed from the image bearing member and a recycled toner conveying path for conveying the recycled residual toner for reusing in the image forming apparatus. Therefore, the background image forming apparatus becomes large in size and brings about an increase in costs due to an increase of the number of parts.
According to a strong demand in the market requiring a high speed performance in operations of color image forming, a tandem color image forming apparatus has been introduced, which is provided with a plurality of image bearing members for respective colors. In such a tandem color image forming apparatus, a plurality of cleaning devices are provided corresponding to the respective image bearing members. However, the tandem color image forming apparatus may be larger in size and more expensive in part cost.
Recently, a charging device employing a charging method using a charging roller has been proposed. In the above-described charging device, the charging roller is held in contact with the image bearing member. In some other charging device, the charging roller is disposed in a vicinity of the image bearing member. There is another charging method also commonly known such as a corotron or scrotron method using corona, for example, which is referred to as a corona discharge method. The corona discharge method causes a corona discharge to charge the surface of the image bearing member. However, the corona discharge method needs a large amount of corona discharge so that the surface of the image bearing member is charged to a desired potential. The corona discharge produces hazardous products such as ozone and NOx that adhere to the surface of the image bearing member, causing an image defect such as image deletion. On the contrary, the charging roller produces a lesser amount of hazardous products.
To reduce the size, the image forming apparatus may apply a cleaner-less system. For example, a technique has been proposed such that an image forming apparatus uses a developing device provided therein for collecting residual toner remaining on a surface of an image bearing member. This technique is referred to as a developing and cleaning method. The developing and cleaning method utilizes the developing device, which functions as a developing device at the same time as a cleaning device. With the developing and cleaning method, the image forming apparatus does not need to include an additional cleaning device. Therefore, the developing and cleaning method can contribute to reduction in size and cost of the image forming apparatus.
However, the image forming apparatus employing the developing and cleaning method and the charging roller method may allow the residual toner to contact a charging member when the residual toner remaining on the image bearing member is conveyed to the developing area. When the residual toner contacts the charging member, it adheres on a surface of the charging member to disturb a charging onto the image bearing member, so that the charging cannot provide a surface of the image bearing member with a desired potential or may cause a charging failure such as a charging nonuniformity. Consequently, an image defect including deterioration of image density and a background contamination may occur in producing an image.
Several attempts have been made to use a developing bias for the purpose of collecting residual toner. The developing bias is applied in a non-image forming operation as well as in an image forming operation, to collect residual toner remaining on a surface of an image bearing member. During the non-image forming operation, a paper jam is recovered, for example.
The above-described attempts, however, may fail to sufficiently collect the residual toner and, at the same time, may cause a charging failure such as a charging nonuniformity, which leads to a defective image having deterioration of the image density and a background contamination.